The invention described herein relates generally to structures and a method for determining the content and distribution of a thermal neutron absorbing material within an object and more particularly to structures and a method for determining the lithium content and distribution within a space reactor heat pipe.
It is useful to know whether the chemical composition of an object varies or is uniform. Using x-rays to make this determination is not always satisfactory. The present invention provides for determining the content and distribution of a thermal neutron absorbing material within an object. The present invention is also concerned with determining the content and distribution of a thermal neutron absorbing working fluid, and lithium in particular, within a space reactor heat pipe.
A space reactor heat pipe is designed to transfer heat energy from a space reactor. The heat can be transferred to power conversion equipment. The heat generated by the reactor can be converted to electricity which is used to power satellites or deep space vehicles. In such applications, the reliability of the heat pipe is critical.
A heat pipe is a closed chamber containing a wick that is saturated with a volatile working fluid. Heat is applied to an evaporator section of the pipe, causing the working fluid to evaporate and move to a condenser section of the pipe, where the vapor condenses and heat is transferred from the pipe. The condensed fluid then returns along the wick by capillary action to the evaporator section.
Heat pipes can have several thousand times the heat transfer capability of the most conductive metals. The ability of a heat pipe to transfer heat can be increased by including an artery with the wick. The artery has a radius greater than the pores in the wick. This increased radius permits a greater flow rate for the condensed fluid.
The ability of a heat pipe to efficiently transfer heat partly depends on the surface tension of the working fluid. If there are gaps in the working fluid in the wick or artery, the pressure difference available to drive the return of the working fluid from the condenser section to the evaporator section of the heat pipe will be greatly reduced. Consequently, it is important to ensure that the heat pipe is uniformly loaded with a working fluid.
Lithium is commonly employed as the working fluid in heat pipes. It is not practical to determine the lithium distribution using x-rays. The lithium is too light and too thin compared to the space reactor heat pipe, which is typically made of molybdenum.
One known method of determining the lithium distribution within a space reactor heat pipe involves neutron radiography, using neutrons emitted from a nuclear reactor. Neutron radiographs have excellent resolution and provide information on the relative lithium distribution, but they do not show exactly how much lithium there is in each segment of the heat pipe. There are other disadvantages to using neutron radiography. For example, the heat pipe must be transported to a conventional nuclear reactor. It is possible for the heat pipe to be damaged on the return trip and such damage would not be revealed by the radiograph. A long exposure to the reactor is required and this entire procedure is expensive. Also, the radiograph only provides information on the lithium distribution before the heat pipe is used. If problems occur during testing of the heat pipe, the lithium distribution can not be checked while the heat pipe is in operation. The heat pipe must be sent back to the conventional reactor to determine whether there is a problem with the lithium.
U.S. Pat. No. 3,577,158 to Hahn discloses an apparatus and method for measuring the mass flow rate of a fluid having a high scattering cross section for neutrons having an energy higher than that of thermal neutrons. In this device many of the higher energy neutrons are slowed to lower energies by a fluid flowing in a conduit. Hahn also discloses a detector to detect the lower energy neutrons. The Hahn device and method of utilizing it are distinguishable from the present invention in several ways. The Hahn device determines the mass flow rate of a fluid. The present invention determines the content and distribution of a thermal neutron absorbing material within an object. More importantly, the Hahn device measures a property of a fluid having a high scattering cross section for higher energy neutrons, whereas the present invention measures the location and amount of a material that absorbs thermal neutrons. Both devices rely on the detection of thermal neutrons, but the Hahn device relies on the number of higher energy neutrons that are thermalized and the present invention relies on the number of thermal neutrons absorbed by a material.